Treatment of natural gas



Tngl @N ORE ATTORNEY F. E. GILMORE TREATMENT OF NATURAL GAS Filed Aug.'7, 1944 July 12, 1949.

Patented July 12, 1949 4aiu-,951

TREATMENT OF NATURAL GAS Forrest E. Gihnore, Bartlesville, Okla.,asslgnor to Phillips Petroleum Company, a corporation ot DelawareApplication August 7, 1944, Serial No. V548.475

2 Claims. 1

This invention relates to the treatment of hydrocarbon gases. In one ofits more specific aspects it relates to the treatment of hydrocarbongases for the removal of nonhydrocarbon gaseous constituents. v

It has been found that some natural gases as produced containappreciable amounts of gaseous impurities such as carbon dioxide,nitrogen or hydrogen sulfide. A gas containing as much as to 20 per centimpurity presents serious considerations when marketing is contemplated.The presence of carbon dioxide and nitrogen lowers the heating value ofa gas merely by their presence while such a material as hydrogen sulfldecauses the gas to be corrosive and to possess a foul odor. Combustionproducts of the latter possess a disagreeable odor as well as beingcorrosive especially when moist. In appreciable concentrations hydrogensulfide is poisonous. In view of these and other considerationstheremoval of hydrogen sulde from natual gas to be used in domesticheating, process Work or metallurgical work is imperative.

Carbon dioxide and nitrogen affect a natural gas for the most part onlyby dilution since both these materials are substantially inert in acombustion zone. Transportation of such a natural gas by pipelinepresents many difficulties and problems especially when the economics ofthe problem are considered. For example, when transporting, say100,000,000 cubic feet per day of a gas containing 10% by volume ofnitrogen through a long pipeline, the operation involves repeatedcompression of- 10,000,000 cubic feet of` an inert gaseous material andthe construction of the pipeline with a capacity 10 per cent greaterthan would be necessary were the nitrogen removed. The cost of suchcompressions may amount to hundreds of thousands of dollars per year andthe additional cost of the pipeline may be greater than the cost of aplant to remove the nitrogen. In addition, the diluting effect of such agas materially lowers the heating value, as for example, a gas having acaloric value of say 1050 B. t. u. to approximately 945 B. t. u. Naturalgas must usually be maintained as 1000 B. t. u. gas, in which case gascontaining nitrogen must not be completely stripped of its condensiblehydrocarbon content, or a naturally lean gas may have to be enriched.

It is suggested that a natural gas having a relatively high nitrogencontent may be treated for removal of this inert nitrogen therebyupgrading the heating value. In case such a gas contains hydrocarbons ofthe gasoline boiling range or is wet as termed by the art, thesehydrocarbons may at least in part be extracted from the gas in the formof natural gasoline, and upon substantial removal of the inert nitrogenstill leave a natural gas of suiiiciently high or satisfactory caloriilcvalue.

(Cl. (i2-175.5)

It is one objectl of this invention to provide an economic process-forthe removal of relatively large amounts of nitrogen from natural gas.

Another. object.of this invention is to provide a process for thesimultaneous removal of inert nitrogen and extraction of condensiblehydrocarbons from a natural gas containing these materials.

Still anotherv object -ot this invention is to provide a process for thesimultaneous removal of inert nitrogen and extraction of condensiblehydrocarbons from a. natural gas containing these materials and stillleave a natural gas of sufciently high caloriflc value and withoutsubstantial loss of the main component of the treated gas. l

Still other objects and advantages will be apparent to those skilled inthe art from a careful study of the following disclosure.

Figure 1 illustrates one form ofl apparatus in which the process of myinvention may be practiced. and I do not wish to be limited therebysince this one embodiment is merely exemplary of the broad aspects of myinvention.

Figure 1A is an illustration of a second method for reuxing of thefractionation tower.

Broadly speaking, my process involves removal of condensiblehydrocarbons from a natural gas, dehydration ofthe remaining gas, andremoval of a further quantity of condensible hydrocarbons. The resultantdry gas is compressed and chilled to such a temperature that methanecondenses to a liquid. This liquifled methane contains some nitrogen andis fractionated to separate the nitrogen from the methane. Methane, asliquid, is used as a refrigerant to assist in liquefying the methane inprocess and is in turn evaporated and warmed. My unique application ofheat exchange steps makes the necessary low temperatures economicallyfeasible.

Referring now to the drawing, raw, impure natural gas containingappreciable amounts of nitrogen reaches my treating system through amain inlet gas line l. The impure gas in line l may come directly from agas producing field or from an intermediate gas storage system, notshown. For purposes of illustration, I will assume that the gas fromline I to be processed is maintained at a pressure ranging from poundsto 1000 pounds per square inch, as for example, about 250 pounds persquare inch and at atmospheric temperature. Natural gas, as produced, inaddition to condensible and noncondensible hydrocarbons usually containshydrogen sulfide, carbon dioxide, moisture and some mercaptan sourness,as well as inert materials such as nitrogen or in exceptional caseshelium. These impurities occur in natural gases in amounts varying fromsubstantially none or traces in many gases to appreciable proportions inother gases; In exceptional gases the nitrogen or carbon difor purposesci illustration I will discuss my invention as appliedy to thepurification of `a natural hydrocarbon gas containing from say to 25%gaseous nitrogen, and in the speciflc case described the gas containedby volume of this inert gas. From the field line I the raw, impure gasma 'be compressed, if necessary, by a compressor 2 to increase thepressure of the inlet gas to a favorable processing pressure. From thecompressor the gas passes to a cooler 3, thence to a gas-to-gas heatexchanger 4 in which the gas is cooled to about 32 F. by indirect heatexchange with` previously cooled and processed gas. From this exchangerthe cooled gas passes to a liquidgas separator 5 in which condensateformed in the exchanger may be separated from the uncondensed material.Much of the sourness, CO2 and moisture are dissolved in this hydrocarboncondensate or condensed simultaneously and may be removed from the gastreating system upon withdrawal of condensate from separator l throughdraw line 6 and passed to disposal, not shown. The uncondensedhydrocarbons and gaseous impurities pass from the separator 3 by a line1 to a treater apparatus 3 in which such impurities as hydrogen suldeand carbon dioxide are removed. This treater may well be one utilizing aconventional process or processes, and the H2B and CO2 may be'removedtogether or separately as desired. From this treating apparatus 3 theC02 and HaS free gas passes by way of a line 9 to a dehydrator I0. Thisdehydrator may likewise, be apparatus of conventional design employing aconventional process and may consist of two or more vessels foralternate on-stream and regeneration as found necessary. The moisturemust be eillciently removed from the gases to prevent hydrate formationin subsequent low temperaturel steps of the process.

The well dried hydrocarbon gases leave the de-I hydretor by a une nwhich une branches and permits a portion of the gas stream to passthrough a gas cooled condenser I2 and the re- '4 f Condensedhydrocarbons which accumulate in the bottom of the condenser I4 are coldand may well serve as a cooling medium. Accordingly,

these liqueiled hydrocarbonsv are removed by a maining portion to passthrough a liquid cooled condenser I3. After emerging from these con.

densers the two portions of the gas stream are combined and enter thelower portion lof a refrigerated condenser I4. This latter condenser maybe substantially any type of condenser, provided, of course, it is ofsuitable design and construction for such low temperature service asherein contemplated.` The gas upward through the tube section II ofthisl condenser which is cooled to the low temperature of from -30 to 1100" F. by an independent refrigeration unit. In this unit such a lowboiling refrigerant as ethylene, for example, may be used. In theethylene cycle, the ethylene previously liqueed, acquires heatindirectly from the hydrocarbon gases being cooled, boils andevaporatesln the tube section I 5 of exchanger I4. The ethylene vaporsare then withdrawn by a line Il and pass through a compressor Il, acondenser I8, from which latter the liquefied refrigerant passes into anaccumulator or surge tank I3. From the surge tank the said liquefiedrefrigerant passes by a return line 20 to a cooler 2| and thence intothe lower portion of the refrigerated condenser I4 to complete theethylene cycle. Makeup ethylene may beyadded through a line 22 on thesuction side of the compressor I l.

line 23 the flow being controlled by the liquid level apparatus 32 andthe motor valve I3 into the liquid to gas exchanger I3 to chill aportion of the feed stock previous to entrance into the refrigeratedcondenser I4. This liquid material after giving up a portion of itscoldness or after being somewhat'warmed in the liquid cooled condenserI3, passes by way of a line 24 into a stabilizer 2l which may be ofconventional design but' suited for low boiling hydrocarbonstabilization. Overhead 'gases from this stabilizer exit by way of aline 2B, pass through a condenser 21, and the condensate in which atleast some propane is condensed accumulates in reux accumulator 23.Liquid level is controlled by a level controller 29 which operates amotor valve 30 to permit return of liquid reflux to the top tray of thestabilizer 25.

Uncondensed hydrocarbons from the accumulator 23 pass by way of a line3| to a main nitrogen free gas ,line 32 which conducts the finallytreated and purified gas from my treating system to be further fullyexplained to a pipeline for transportation to a market or to a gasholder for storage, not shown, or to other disposal as desired.

Stabilized natural gasoline passes from the base of the stabilizerthrough a line 54 to storage or other disposal, as desired.

The uncondensed gases consisting of nitrogen and methane withsubstantially no ethane or heavier hydrocarbons issue from therefrigerated condenser I4 through a pipe 33 into an auxiliary chiller 43and finally are passed through still another exchanger or condenser 34wherein a major portion of the methane is condensed to liquid methaneand most of the nitrogen, of course, remains as a gas. This mixture ofgaseous nitrogen and liquid and gaseous methane passes from the saidcondener 34 by way of a pipe 35 into a fractionator tower 38 hereintermed the denitrogenizen This vessel may usually be a conventionalbubble cap fractionator. but so designed and constructed as to operateat low temperature and high pressure as required by such service as'herein discussed.

The fractionator .or denitrogenizer 38 is equipped with conventionaltype bubble cap'trays,

. a reflux apparatus 3l and a reboiler coil 33. As

mentioned above, charge stock to this column contains largely liquidmethane, some gaseous or uncondensed methane, nitrogen and any otherdiiliculty condensible material having a boiling point near that ofmethane or below which has not been 'previously removed. Liquid methanecontaining some dissolved nitrogen descends through the fractionator thenitrogenbeing fractionated out in the descent until the liquidaccumulating in the reboiler section is almost pure methane. Reboiling,coil 33 furnishes heat to boil this methane to produce the desiredrectication in the fractionator. Heat for said reboiling is furnished bypassing a small portion of the partially chilled methane and nitrogenfrom the line 33 through abypass line 39 into the reboiler coil. Theexhaust from the reboiler coil is passed through a pipe 4I and isdischarged into condenser 34 with the gases from the chiller The upperportion of the denitrogenizer is for the most part constructed in amanner similar to that of conventional fractionators. A pipe 42 removesoverhead vapors and gases from the fractionator, which material *n mycase consists mainly of gaseous nitrogen and uncondensed methane. Thismaterial passes through the line 42 to a heat exchanger 43 and afterbeing warmed it is compressed by a compressor 44 and cooled in a cooler45. The stream issuing from cooler 45 is split. one portion passingthrough exchanger 43 and the other portion passing through an exchanger46. These two stream portions issuing from said exchangers combine in aline 41 and arc passed to a reflux accumulator vessel 48. In this vesselmethane which has been condensed in exchangers and 46 separates from thegaseous nitrogen and some uncondensed methane. The nitrogen anduncondensed methane are removed from the top of this accumulator by apipe 49, expansion being permitted by an expansion valve 50 set tooperate according to a desired pressure in accumulator 48. -Therefrigeration available from this expansion absorbs heat in theexchanger 46 to chill and condense lsome methane from themethane-nitrogen mixture which passes in indirect heat exchangetherewith. The expanded nitrogen-methane mixture then issues' from aline 5I for such disposal as desired. I prefer to operate the plant insuch a manner that this off gas contains from 25 to 50% nitrogen, theremainder being methane, and in this manner sumcient methane isavailable for plant power purposes.

The liquid methane which has accumulated in the accumulator 48 isreturned to the top of the denitrogenizer by a line 52 to serve as a wetreiiuxing material. The level of the liquid in accumulator 48 may becontrolled by a motor operated valve 53 actuated by a liquid levelmechanism on the accumulator tank. Upon opening of the motor controlledvalve 53 the liquid methane passes therethrough from the pressureoriginating in the compressor 44.

A thermoregulator assembly 55 which controls the amount of flow of coldgas in line 39 to the reboiler 38 is actuated in response to thetemperature of the liquid in the base of the column 36. Thissubstantially nitrogen-free liquid methane is passed from the base ofthis column through a line 56 in which is placed a motor operated valve6| actuated by a liquid level mechanism on the denitrogenator 36, to thespace around the tubes of the heat exchanger in exchanger vessel 34. Inthe space around these tubes much or all of the liquid vaporizes under alower pressure than the gas inside the tubes imparting its lowtemperature to the higher pressure methane inside the tubes by whichoperation the latter at least in part condenses. The Vaporized methaneissues from the refrigerated condenser by a line 51 and is led to theexchanger 40. From this exchanger the methane passes by way of a line 58to still another exchanger 2l, thence by a line 59 to yet anotherexchanger I2, previously termed the gas cooled condenser. From thiscondenser the methane gas passes by a line 69 into the first mentionedgas to gas exchanger 4, thence by line 12 into the main product line 32.

The liquid hydrocarbons, that is, those more easily condensed, areremoved by line 23 and the flow in which is controlled by a motor valve63 in the line 24 which is actuated by a liquid level apparatus 62. Thismotor valve serves as a back pressure regulator on the liquid cooledcondenser I3 to prevent undue evaporation of the-cooling medium withinthe condenser. It also restricts or controls the flow of condensedhydrocarbons into the stabilizer tower 26. A pump 64 serves to assist intransfer of these condensed hydrocarbons from the condenser I3 into thestabilizer according to the level of the refrigerant in' accumulator I9to control the rate of flow of said refrigerant by actuation of a motorvalve 65 in a line 68. rThis refrigerant upon passage through valve 65evaporates as heat is absorbed from gas passing through the condensingtubes i5. Refrigerant gas exits through the line I6 and is recompressedby compressor I1, as mentioned hereinbefore. Makeup refrigerant may beadded through the line 22.

In the operation of the process according to my invention I contemplatetreating iiD-100,000,000 cubic feet of a natural gas containingapproximately 15% nitrogen by volume, a normal amount of sulfursourness, carbon dioxide and some moisture.

'Ihe denitrogenizer or fractionator tower may be operated under dryreflux in place of the wet trim as mentioned above. In this case theoverhead nitrogen-methane gas is expanded through a needle valve toalmost atmospheric pressure the cold expandedgas passing through aclosed reux coil in the top of the tower. This very cold coil issufficiently cold to condense considerable of the methane which latterthen flows down the tower as a liquid reflux. To assist further in thedry reiiuxing ofA this denitrogenizer tower, the apparatus and iiowillustrated in Figure 1A may be used. The above mentioned fractionatoroverhead nitrogen-methane gas line 42 connects to an expansion valve 15which permits passage of expanded gas through the closed reflux coil 16.This cold expanded gas passes from the coil through an exchanger 11 as arefrigerant for cooling another refrigerating medium. This effluentnitrogen-methane gas may be-used as the refrigerant in a supplementaryunit to assist in the very low temperature closed coil refluxing of thisfractionator. However, if desired, another refrigerant may be used, asillustrated inthe Figure 1A. Referring to the figure this refrigerant iscompressed in a compressor 18 and cooled in a cooler 19. The cooledstream passes through a line 8l)l and is divided into two portions asrepresented by lines' 8l and 82. Line 8l conducts a portion of thisgaseous refrigerant to the above mentioned exchanger 11 while theremaining portion in line 82 passes through another exchanger 83. Bothstreams of the refrigerant are cooled and may be partially or whollycondensed on passing through these exchangers, and again join in a line84, pass through an expansion Valve 85 and thence into an auxiliaryclosedrefiux coil 86. This coil is preferably disposed below the coil16. The expanded eflluent from the coil 86 passes through the exchanger83 in which it cools and may condense the refrigerant enroute to thereflux coil 86. If the top fractionator pressure is about '100 poundsper square inch or higher, the Joule-Thompson effect of the very coldexpanding gases will refrigerate coil 16 to a low enough temperature tocondense a large portion of the methane and the methane-nitrogen mixtureleaving coil 16 may be utilized for fuel for power and heat needed forthe operation.

The refrigerant, either the methane-nitrogen off gas or anotherrefrigerant, .is compressed by compressor 18 to a pressure'of from 1000to 5000 pounds per square inch. Upon being cooled in cooler 19 andfurther cooled in exchangers 11 and 83, and when expanded in valve 84produces sufficiently low temperature in coil 86 to con-l dense methanesand'y even nitrogenif desired.

Since such an extraction plant as that discussed herein consumesconsiderable power, I believe it preferable and advisable to permit arelatively large concentration of methane to pass off the top of thefractionator with the nitrogen as mentioned hereinbefore. In thismanner, the nitrogen can be more efliciently removed from the field gasand at the same time at less cost. Since it is a difficult and expansiveoperation to remove all or substantially all the methane from thenitrogen in the fractionator my proposed operation is relativelyeconomical. contemplate to control the nitrogen containing off-productrelative to its methane content in such a manner as to furnish all orsubstantially all the methane needed for heat and power purposes in theprocess.

For carrying out such a process as herein disclosed, it is needless tosay' that all pipes, ex-

In fact, I

changers, valves, etc. which carry low temperature liquids or gasesshould be well insulated by the best thermal insulation obtainable. Itmight vbe advisable to install all the cooling that the individualpieces of equipment need not be so heavily insulated. All operatingcontrols, indicators, and instruments should be outside this building,the temperature of which should preferably be maintained as low as 0 F.-or below.

The expansion valves, pipes, all vessels and all parts and members ofthis low temperature plant should be made of such materials and sodesigned as to withstand the very low temperaturesV of operation.Similarly, the equipment should be capable of withstanding all pressuresnecessary,

compressing and cooling the gas, dividing said compressed and cooled gasinto two portions, chilling one portion by a rst indirect heat exchangewith a refrigerated Vaporized methane, chilling the other portion by asecond indirect heat exchange with refrigerated liquid methane,combining these two portions and further chilling the combined streamAby a third indirect'heat exchange with vaporizing liquid methane toproduce some liquid methane containing dissolved nitrogen and leavinguncondensed most of the nitro' gen and some gaseous methane,fractionating the chilled combined stream of liquid methane containingdissolved nitrogen and gaseous nitrogen'- and methane to produce aliquid methane bottoms product and an overhead gaseous product ofnitrogen and methane, partially condensing this gaseous product andadding the condensate to the equipment within a well insulated buildingso 1 fractionating step as liquid reflux, and removing the nitrogen andsome Auncondensed methane as a. product of the process; adding reboilingheat to the fractlonator liquid bottoms by the second indirect heatexchange thereby producing liquid methane free from dissolved nitrogenand termed refrigerated liquid methanevaporizing this refrigeratedliquid methane inthe third heat ex` change thereby producing arefrigerated Vaporized methane,warming this refrigerated vaporizedmethanein the first heat exchange thereby producing warmed vaporizedmethane, and removing thlswarmed vaporized methane as the major productof the process.

2. A process for purifying natural gas containving gaseous nitrogen asan impurity, comprising, compressing and cooling the compressed gas, di-Viding the compressed and-cooled gas into two portions, chilling onelportion by a first indirect heat exchange with a chilled `gaseousmethane, subsequently produced, chilling the other'portion by a secondindirect heat exchange with al previously refrigerated liquid methanefraction ator bottoms, subsequently produced, combining these twochilled portions of compressed `gas into A one stream and furtherchilling this stream by a third indirect heat exchange with evaporatingliquid methane, whereby some of the methane of the gas stream iscondensed and dissolves some gaseous nitrogen, and passing as feed stockthis stream of vliquid methane containing dissolved nitrogen and gaseousnitrogen and methane into a fractionation zone at a point intermediatethe ends thereof and therein fractionating saidfeed stock to produce aliquid methane bottoms free from dissolved nitrogen and an overheadgaseous product containing nitrogen and some methane; compressing andchilling said overhead product to produce a liquid condensate anduncondensed nitrogen containing some methane, adding said condensate tothe top of said fractionating zone as liquid reflux, and removing saiduncondensed nitrogen containing some methane as a product of theprocess; adding reboiling heat to the bottom ofthe fractionation zone bysaid second indirect heat exchange, vaporizing said liquid methane insaid third heat exchange, and warming said vaporized methane in said rstheat exchange, and removing the warmedV vaporized methane from saidiirstheat exchange, as the purified natural gas.

FORREST E. GILMORE;

- 'REFERENCES CITED The following referefnices are of record in the iileof this patent: i

y UNITED STATES PATENTS Number Baufre Apr.v 21,. 1942

